Electromechanical Diesel Engine Starter


This is a description of an electromechanical diesel engine starter that is meant to control an engine with a 24V starter and a shutdown solenoid (be it on the governor/fuel rack, air gate or fuel supply).

A set of local start/stop pushbuttons is provided with the starter. Remote start/stop pushbutton stations can also be used, as can a solid state engine controller. Starting/stopping is through the industry standard method of NO start buttons and NC stop buttons. An adjustable time delay for the shutdown solenoid is provided so that the stop pushbutton need only be depressed momentarily.

The drawing Electric_DselStrt can be downloaded as a PDF. In this drawing, the schematic can be divided into two sections. On top are the circuits that control starting and stopping of the diesel. Below are the circuits that power the radiator fan as well as a heater for the generator's windings.

Starter Operation

Assuming that the starter has just been powered up or was previously in the off state, this is its sequence of operation.

To start the engine, either the local pushbutton (SW11) or one of the remote pushbuttons or engine controllers attached to TB2 is closed and held closed while the engine cranks (if there are no remote pushbuttons or controls, the +24V wire from the battery, marked GPU2/+, should be connected directly to TB1, otherwise GPU2/+ feeds all of the remote start pushbuttons and/or other controls). This closes the starter contactor (CT9), which in turn energizes the starter solenoid (SS), thereby cranking the engine. At the same time, run relay (RY12) and shutdown relay (RY13) are energized through the diodes and both latch.

Once the engine starts, the pushbutton is released or the engine controller removes power from TB2. The starter contactor and solenoid are released and cranking ceases. The engine keeps running because, well, its a diesel. As long as it has air and fuel, its going to run. The hour meter, which is connected to the shutdown relay, is energized and accumulates time.

If the local pushbutton (SW12) or any of the remote pushbuttons or engine controllers wired to TB3 cause power to be removed, the latching circuit of shutdown relay (RY13) is broken and the relay is deenergized. This applies power to shutdown contactor (CT10) and shutdown timer (TD7) through the NC contacts of RY13 and the NO contacts of RY12. The air gate is closed, the fuel rack is moved to the no fuel position, etc. by the shutdown solenoid, which remains energized while TD7 times out.

Once the time delay of TD7 (typically 30 seconds) runs out, TD7 is activated, which breaks the holding circuit of RY12. Relay RY12 opens, deactivating itself, TD7 and CT10 in the process and releasing the shutdown solenoid. The entire circuit is now ready for the next cycle.

Note that the time delay of TD7 should be set long enough to ensure that the engine shuts down plus a little time for good measure. However, it should be noted that some solenoids are not meant to be activated for long periods of time so you cannot just set the time delay for two or three minutes. A reasonable margin of safety (e.g. 10 seconds) should be added to the actual shutdown time required.

Radiator Fan

In the bottom half of the circuit, we see that the radiator fan motor is powered by the generator itself, through a circuit breaker and contactor (CT11) which is part of the motor starter. Power for this contactor is supplied through the starter's overload contacts and the fan thermostat, which is located in a suitable place to measure the engine coolant return temperature. When then coolant reaches a reasonable temperature and, ergo, the engine reaches its proper operating temperature, the thermostat closes and the fan runs.